Fabrication And Structural Design Of Aluminum Borate Based Ceramic Materials With Integration Of Neutron Shielding And Thermal Insulation

Nuclear material technology is not only the basis for the safe operation of the existing nuclear energy system,but also a significant prerequisite for the design and development of novel reactors.As a nonnegligible key part of nuclear materials,nuclear thermal insulation materials could reduce the heat loss in the transmission process of nuclear high-temperature working medium,thus greatly improve the energy utilization efficiency and corresponding economic benefits.In view of the present widely applied nuclear thermal insulation materials such as fiber-based products and metallic materials,large amount of radioactive dust guaranteed during the replacing of the former could detriment the health of personnel,the complicated fabrication process and high cost of the latter limit their further popularization and application.Besides,higher requirements on nuclear materials have been put forward for the ever-evolving nuclear power system,the intermediate neutron shielding performance was given to the existing nuclear thermal insulation materials based on the concept of"integrated design of structure-multifunctional",which means the research of integrated materials of thermal insulation and neutron shielding.This could ensure the structural stability of transmission pipeline for high temperature working medium in nuclear energy system under high-dose irradiation,which is of great significance to further application value.In this work,aluminum borate based ceramics are regarded as the main research object which possess excellent comprehensive performances such as high strength,low density,low thermal conductivity and neutron shielding.These characteristics could render aluminum borate based ceramics extremely as the ideal matrix materials for integrated design of neutron shielding and thermal insulation.On this basis,the phase transformation,microstructure evolution and structure-property relationship of aluminum borate based ceramics were systematically explored with the research ideas of"reactive self-bonding,liquid phase sintering,multi-phase enhancement and light-weight design".And aluminum borate based foams with excellent neutron shielding and thermal insulation performance were fabricated and further designed with gradient structure.Then,the structure-property correlation mathematical model of the intrinsic property and radiation shielding performance was established.Finally,the integrated design of neutron shielding and thermal insulation was achieved.Based on above mentioned works,the main conclusions could be drawn as follows:(1)Aluminum borate ceramics were fabricated by reactive self-bonding and the reaction paths were explored.The raw materials,heating temperature and Al/B molar ratio are all significant factors affecting the structure and properties of reactive self-bonding aluminum borate ceramics.Compared with utilizing?-Al₂O₃/H₃BO₃as raw materials,using?-Al₂O₃/2Al₂O₃·B₂O₃micro-powder could not only avoids the decomposition of H₃BO₃at low temperature which could generate lots of pores,but also eliminates the surplus?-Al₂O₃phase,which could reduce the thermal expansion coefficient and thermal conductivity of ceramic skeleton on the basis of enhancing the mechanical properties of the samples.The effects of the dispersive density of boron element and thickness on the neutron shielding performance of the sample was explored based on the power-function transformation of Beer-Lambert formula.The feasibility of using aluminum borate ceramic as the basic material for the integrated design of neutron shielding and thermal insulation could be proved.(2)Introducing Bi₂O₃which is regarded as"functionalized sintering assistant"into aluminum borate ceramics.Bi₂O₃could endow aluminum borate ceramics?rays shielding capacity on the basis of reducing sintering activation energy,which is also the premise for the subsequent introduction of Gd-containing neutron shielding agents.Doping 3 wt%Bi₂O₃could drastically reduce the sintering activation energy by about 50%,and the effect of Bi₂O₃in promoting the sintering densification of samples was more notable than the volumetric expansion effect of reactive formation of9Al₂O₃·2B₂O₃.The influence of the dispersive density of boron/bismuth elements and sample thickness on the neutron/?rays shielding was also investigated.(3)Multi-phase enhancement of neutron shielding performance could be achieved based on reactive forming Gd BO₃into aluminum borate skeleton.Selecting matrix material with lightweight,high-strength and?rays shielding could enhance the mechanical properties of the composite ceramics on the basis of partly avoiding?rays generation of Gd by neutron excitation.The introduction of Gd₂O₃could result in the re-appearance of?-Al₂O₃in the system,and the distribution of reactive formed Gd BO₃in the material is unique textured microstructure with lots of micro-cracks near the crystals.The high-temperature structural stability of composite ceramics was experimentally tested and in detail numerically simulated via finite element method.Adding Gd₂O₃could effectively improve the neutron shielding performance of the samples and the maximum neutron shielding rate of the composite ceramics is 82.8%.(4)The controllable lightweight preparation of aluminum borate based foams could be achieved,and further applied structural gradient design.The controllable fabrication of reactive self-bonding aluminum borate foams and aluminum borate-gadolinium borate composite foams could be carried out by adjusting the process parameters which affect the rheological properties of ceramic slurries.With porosity increased,the pore size distribution gradually changes from unimodal to multimodal distribution,and the average pore size increases significantly.Increasing heating temperature could make micro-cracks near the Gd BO₃crystals in the struts of composite foams gradually disappear and further transform into an interlocking structure of9Al₂O₃·2B₂O₃and Gd BO₃,which could effectively enhances their specific strength.COMSOL Multiphysics software and neutron shielding tests were combined and proved that the gradient design of ceramic foams could not only effectively enhance the thermal insulation efficiency,but also possess excellent neutron shielding performance and mechanical strength.The obtained results in this paper could support the idea of“integrated design of neutron shielding/thermal insulation”.